What equation should we use to fit the graphs of speed vs. std dev?

In particular, what should we use to account for the curve at low speeds?
Simple combo of spatial, temporal uncertainty = sqrt(sqr(spatial) + sqr(temporal*speed))

This paper (Ramamurthy, M., Bedell, H. E., & Patel, S. S. (2005). Stereothresholds for moving line stimuli for a range of velocities. Vision Res, 45(6), 789-799) has some interesting modeling and refs to earlier modeling that should be helpful.

Most of the moving Vernier papers (including all the SC Chung and Bedell papers I think) don't temporally ramp their stimulus! Only exception I know is
Mechler & Victor who ramped just over 20 ms Mechler, F. & Victor, J. D. (2000). Comparison of thresholds for high-speed drifting vernier and a matched temporal phase-discrimination task. Vision Res, 40(14), 1839-1855.
Absence of ramp could easily reduce the degradation caused by motion and invalidate the model's predictions for uncertainty based on velocity. None of the papers seem to address this point.